1. Field of the Invention
Embodiments of the invention generally relate to distributed temperature sensing.
2. Description of the Related Art
Distributed Temperature Sensing (DTS) enables monitoring temperature along the length of a well bore, for example. DTS employs an optical fiber installed along the length of a well to function as both a communication line and a temperature sensor. A laser or other light source at the surface of the well transmits a pulse of light into the fiber. As the light propagates through the fiber, scattering reflects some of the light back towards the surface for detection. In Raman scattering, incident light is scattered by optical phonons and undergoes relatively large frequency shifts. In Brillouin scattering, incident light is scattered by acoustic vibrations (phonons) and undergoes relatively small frequency shifts. The frequency or intensity of these reflections relative to the pulsed light shift in accordance with the temperature of the atoms along the fiber. Accordingly, processing of this reflected light as a function of time can derive temperature as a function of well depth, with earlier reflections indicating the temperature at shallow depths, and later reflections indicating the temperature at relatively deeper depths.
However, assessment of the depths based on time of travel of the light relies on various assumptions that can vary from fiber to fiber and create uncertainty as to locations of the temperatures measured. The assumptions result in potential significant error making reliable accurate determinations of where points of a temperature profile from the DTS correspond difficult. Application specific requirements and conditions like cable design, deployment methods and inaccessibility of the fiber further complicate relating actual physical location of a given point along the installation to distance in a DTS measurement.
For example, assumptions may relate to refractive index of the fiber and/or overstuff of the fiber within a cable or other system component. Distance computations based on the time of flight for the light depend on the refractive index of the fiber through which the light propagates. Utilizing an average refractive index associated in general with material properties of the fiber fails to account for real variances in the refractive index that may occur at different positions along the fiber, particularly if different lots of fiber are used in different cable sections, or any changes in the refractive index over time. Further, unknowns regarding amount of an excess length of the fiber within a relatively shorter length of the cable necessitate estimation to correlate between computed distances along the fiber and the length of the cable.
Therefore, there exists a need for improved systems and methods of distributed temperature sensing.